In recent years, there has been a growing trend toward a smaller size and larger capacity magnetic disk drive. To deal with this trend, the unit recording area of a magnetic disk is decreased, while the storage capacity per disk is increased. Since the magnetic signal becomes weaker with a decrease in unit recording area, the detection sensitivity needs to be improved. Therefore, the flying height of a magnetic head should be reduced further, which in turn requires that the surface roughness or waviness of a magnetic disk substrate (also referred to simply as “substrate” herein) be sufficiently small. Moreover, the magnetic head flies and moves over the entire surface of the substrate, so that it is necessary to reduce the waviness of the entire surface of the substrate. To meet these requirements, a polishing composition including silica particles (abrasive particles) with a controlled particle size distribution has been proposed (e.g., JP 2004-204151 A).
JP 2004-204151 A discloses a polishing composition that uses colloidal silica having a specific particle size distribution. In this polishing composition, the particle size of the colloidal silica is reduced, and the particle size distribution is sharpened, thereby reducing the surface roughness of a substrate for a memory hard disk.
However, when the abrasive particles have a smaller particle size or sharper particle size distribution, it is difficult to reduce the maximum height Rmax of the surface roughness of the substrate that is measured with an atomic force microscope (AFM) and the waviness in the peripheral portion of the substrate without impairing the productivity.